Overview of some recent geophysical investigations in Himalaya

Tectonophysics, 62 (1980) 99-111 @ Elsevier Scientific Publishing Company,

99 Amsterdam

- Printed

OVERVIEW OF SOME RECENT GEOPHYSICAL HIMALAY A

in The Netherlands

INVESTIGATIONS

IN

HARI NARAIN Banoras Hindu Uiziversity Vuranasi, (Received

February

UP. (India)

23, 1979)

ABSTRACT Narain, H., 1980. Overview of some recent J.M. Tater (Editor), The Alpine-Himalayan

geophysical investigations Region. Tectonophysics,

in Himalaya. 62: 99-111.

In:

Recent work on long-period surface wave dispersion investigations and other geophysical work have shown that in the Himalayan and Tibet Plateau region the crust is extremely thick and the velocities are low: However, the upper mantle below Tibet appears to have normal velocities. Seismic Research Observatories, being established in the vicinity of Himalaya, will be extremely useful for near-source investigations due to their digital data acquisition capabilities and much larger dynamic range. Quantitative seismicity maps prepared for the Himalayan region are useful in comprehending regional tectonics. With international collaboration, Deep Seismic Sounding surveys have been sue cessfully carried out in western Himalaya. It is inferred that the northern boundary of the Indian Plate does not lie along the Main Central Himalayan Thrust or the Indus Suture line, but falls very much north of the combined Indo-Tibetan block. Focal mechanism studies are, by and large, consistent with the northward thrusting of the Indian Plate. Conflicting results regarding the prevalence of isostasy in the Himalayan region have been obtained from gravity surveys. Geophysical investigations and observational facilities need to be intensified for a better understanding of the tectonics.

INTRODUCTION

The Himalayan Range constitutes the largest concentration of mass on earth. Recently considerable international effort in the study of earth sciences, p~~cu~~ly geology and geophysics, has been channeled into Upper Mantle and Geodynamics projects. The recent ocean bottom studies, where the geo1ogica.l history over the last 200 m.y. is well preserved, have supported the theory of plate tectonics: However, the continental masses, with much older history going back to Archaean times, are much more complicated, and well-defined evidence for the support of the plate tectonics hypothesis are not very evident. The Pamir Knot, the Tibetan Plateau, the Assam Syntaxial zone and the Himalaya possibly hold the key to the understanding of the geodynamical processes in these regions.

100

Geophysical studies including gravity, magnetic, heat flow measurements and paleomagnetic studies deserve special attention. Seismological studies including both surface and body waves, and fault plane solutions of major earthquakes in the region, have provided a certain framework within which to continue the studies in tectonics. However, the network of seismological stations is meagre and many more quality stations need to be established to ensure a better coverage and understanding. The geophysical studies in the Indo-Gangetic Plain, the qualitative seismicity maps of the Himalayan region at 0.5” grid, the larger coverage of the LANDSATimagery and restricted Deep Seismic Sounding profiles along lake Zorkul in the Pamir (USSR), Nangaparbat (Pakistan) and Srinagar and Pirpanjal (India), have provided the basis to postulate the present plate boundary further north of the Tibetan plateau rather than along the Indus Suture zone, as commonly presumed. SURFACE

WAVE

DISPERSION

INVESTIGATIONS

On the basis of surface wave dispersion investigations carried out more than a decade ago, Gupta and Narain (1967) reported a 65-70 km thick crust characterized by low seismic velocities in the Himalayan and Tibet Plateau regions. This has been reconfirmed through recent results reported by Tung (1975), Knopoff (1976), Bird and Toksoz (1977) and Chun and Yoshii (1977). These investigations suggest a thick crust (70 * 10 km) and relatively low crustal velocities. However, it has been reported by Harsh Gupta at the Erik Norin Penrose Conference on Tibet (Molnar and Burke, 1977) that the upper mantle beneath Tibet has a normal shield structure. Some interesting results have been reported by Gupta et al. (1977a) for a great circle path passing just south of the Alpine-Himalaya range. Fundamental-mode Rayleigh and Love wave group and phase velocities between WWSSN stations at Chiengmai, Thailand; New Delhi, India; Shiraz, Iran; and Helwan, United Arab Republic, have been obtained. Application of an improved frequency-time analysis technique makes it possible to obtain results for periods extending to several minutes. They have introduced the “impulse method for estimating interstation dispersion. This method is response” found to be superior to the usual cross-correlogram method since spectral amplitudes are equalized to a greater degree. Gupta et al. (1977a) report that these interstation Rayleigh and Love wave group and phase velocity results are consistent with a shield-like upper mantle velocity structure. In another detailed study, Gupta et al. (197713) inverted the surface wave dispersion data extending to 200 set between New Delhi, India and Chiengmai, Thailand and found that the Model 5.08 shield (Fig. 1) developed by Hamada (1972) fits the observations very well. Gupta et al. (1976) conclude that the extension of the Peninsular Shield of India below the Indo-Gangetic Plains had been inferred on the basis of field geology evidence, as summarized by Gansser (1974) and Valdiya (1976). However, this geological deduction previously lacked the supporting geophysical measurements which are

101

SHEAR

ti 0

200

( KdSec)

VELOCITY

__---T,

TP-4

-&--+ --u-+--

5 OS SHIELD 5 OS R-TECTONIC 5 OS L-TECTONIC

-

CANSD

’

300

Fig. 1. The 5.08 shear velocity from Gupta et al., 1977).

model

and the CANSD

shear velocity

model

(adopted

provided by their detailed mantle surface wave dispersion analysis. Recently, Seismic Research Observatories (SRO) equipped with bore hole seismometers have been installed at a number of suitable locations (Peterson and Orsinim, 1976). The data acquisition is in digital form and the system has a much bigger dynamic range compared to the conventional WWSSN system. It is expected that SRO data would be extremely useful in investigating larger magnitude earthquakes at shorter distances and thus improving our knowledge of the complexities of the Earth’s structure. SEISM~C~TY BOUNDARY

OF CENTRAL

AND

SOUTHEAST

ASIA

AND

THE INDIAN

PLATE

A quantitative seismicity map of the Alpine-Himalayan belt, including the Sunda Arc region, was prepared by Kaila and Narain (1971) and its position in relation to Asia is reproduced in Fig. 2. It can be seen that the most intense seismicity is aligned along the southern side of the Tien Shan fold belt. This trend takes a southeastward swing passing along the southern margin of Tsaidam Block and the Nan Shan fold belt. It further trends northeastward going up to southeast of Peking. In the east the Tien Shan-Nan Shan seismicity high zone takes a southward swing from Siam and joins Arkan Yoma and Sunda island arcs further south. Another well-demarcated high seismicity zone aligns in the NW-SE direction parallel to the MCT in the Kumaun-Punjab Himalaya, whereas high seismicity zones in NepalBhutan-NEFA Himalaya are aligned transverse in the MCT. In general the

0’

20’

40’

60’ N

6ElSYICITY

CONTOURS

BASED ON

AS POSTULATED

t- - -

SOUNOARY

OWEN FRACTURE ZONE AND 1TS PROPOSED

,ND,AN PLATE MARI NARAIN

BY

AN0

ON LAND

E L UAILA

CONTINUATION

103

seismicity of the Himalayan belt is less severe compared to Tien Shan-Nan Shan belt, and is not entirely confined to MCT but is associated with a number of transverse trends which may extend to Tibet. Kaila and Narain (1976) suggest that Tien Shan-Nan Shan seismicity zone is the northern boundary where the Indian Plate is subducting. They argue that this inference is supported by the absence of intermediate and deep focus earthquakes in the Himalayan region. The western boundary of the Indian plate proposed by Kaila and Narain is formed by a southward continuation of the Pamir-Alay fracture zone passing along the western boundary of the Badak Shan mountains, extending further south to the fault zones of the Sulaiman and Kirthar ranges which further extend to the Owen Fracture zone in the Arabian sea. In the east, the Indian plate boundary divides southeastern China from the rest of Tibet. It passes along Lung Men Shan thrust and the Arkan Yoma fold belt, further continuing southeastward to join with the Indonesia trench system. It is postulated that the combined Indo-Tibet block trends simultaneously northwards and eastwards. DEEP SEISMIC

SOUNDING

INVESTIGATIONS

Under an international collaborative program, Deep Seismic Sounding investigations have been carried out in Pamir Himalaya between 1973 and 1975 by scientists from USSR, India and Italy (working with the Pakistan team). This has been one of the most successful collaborative efforts and preliminary results of analyses for a number of profiles have been published by Aliev et al. (1976), Kaila et al. (1978), Scarascia et al. (1980), Finetti et al. (1980) and Bartole et al. (1980). Finetti et al. (1978) report that the reversed DSS profile from Pamirs across the Karakorum Range and the Nanga Parbat massif to the Indus Platform shows the existence of a 70-km thick crust under the Karakorum Range (Fig. 3) with a velocity of about 8.05 km/set at the Mohorovicic discontinuity. The Mohorovicic discontinuity rises to a depth of about 60 km under the Pamirs. Beneath the Karakorum Range high-velocity crustal layers are present in the first 23 km. These are underlain by a low-velocity interval of 30 km thickness, followed by a 7 km/set lower crust simatic layer. Scarascia et al. (1980) report an increase in crustal thickness, i.e. from 59 km below Lawrencepur to 65 km below Astor located 100 km away. They found the crustal thickness deduced compatible with the gravity data. Bartole et al. (1980) have traced geomorphological lineaments from LANDSAT imageries of the Pamir-Himalayan syntaxis and integrated this information with the gravity and DSS data. On the basis of structural and geophysical data the post-Hercynian area of the DSS profile has been divided into three main geodynamic belts. Aliev et al. (1976) have summarized the results of Deep Seismic Soundings in the Northern Pamirs (Fig. 3). The section between Lake Zorkul and Uzgen is particularly interesting. The isoline of VP = 5 km/set passes at depths from

b

snot

PolNT

0 NEAR

BY

LOCATfON

0

ClTV

Fig. 3. Scheme of shot points along the International Sounding profile.

Pamir-Himalayan

Deep Seismic

1 to 3 km. At depths in excess of 7 km, inhomogeneities in the Earth’s crust and their lateral variations are reported to be more pronounced. In general, four blocks have been identified along this profile. The territory of southern and central Pam&s between lakes Zorkul and Karakul comes under the south Pamir block. The isoline for VP = 6.5 kmfsec is at a depth of 13-14 km in this block, Next comes the Karakul lake block which is characterized by the Pam+-Hindukush zone of high seismicity, and the isoline of VP = 6.5 km/ set deepens in steps to a depth of 19 km. In the adjacent north Pamir block the depth of the 6.5 km/set isoline further deepens to 23 km. The fourth and the northern-most block, where the profile under discussion terminates, is the East Farghana block. Here the VP = 6.5 km/set isosurface shoals to a mere 12 km depth. Aliev et al. (1976) also mention reduced crustal velocities

105

in the middle of the crust under southern Tien-Shan, which can be traced in many other sections. This also is reported by Finetti et al. (1980) and Scarascia et al. (1980). Kaila et al. (1978) have reported the results of this International DSS Project in Kashmir Himalaya. They reported an average down dip of 15” to 20” between Sopur and Kanzalwan (Fig. 3) for the Mohorovicic discontinuity: its depth increasing from about 45 km near Sopur to 54 km near Wular lake, and to 64 km near Kanzalwan. In general, the crustal velocities in the Kashmir Himalaya are found to be lower than those in the Peninsular Shield and the crustal thickness in the Kashmir Himalaya is greater than the shield. However, the upper mantle velocity functions are quite similar in the two regions. This is concordant with the results from surface wave dispersion investigations. A deep reflecting boundary at a depth of about 140 km also has been reported by Kaila et al. (1978) beneath Nanga Parbat to the great Pamir ranges. FOCAL MECHANISM AND STRESS DROP STUDIES

The focal mechanism solutions in the Himalaya are generally thrust with a small component of .strike-slip in some cases. Fig. 4 shows the focal mechanism solution reported by various authors. The fault plane solutions for earthquakes occurring in Himalaya and near the Shillong plateau showed that India is underthrusting the Himalaya. The direction of under-thrusting in this eastern part of the Himalaya is more northerly than in the rest of the FAULT

PLANE

SOLUTIONS

AND

’

0

ADJACENT

.

Thrust

IN

HIMALAYA.

BURMA

AREAS

Faulting

a. 0

Normal

Foultlng

.

Fig. 4. Fault plane solutions in Himalaya, Burma and adjacent areas, obtained by various workers. The dark and white areas indicate the regions of compression and dilatation, respectively. (Adapted from Rastogi, 1976.)

arc. The seismic slip associated with this active zone of thrust faulting has a uniform strike consistent with the northeasterly direction of convergence for the Indian ocean and Eurasian plates computed by Le Pichon (1968). The rate of convergence of approximately 5.6 cm/yr along the mountain front is in close agreement with a seismic slip rate computed from a magnitudemoment relationship (Brune, 1968) applied to major earthquakes that have occurred in the Himalayan region since the beginning of the century. This evidence suggests that the seismic slip accounts for the convergences between the Indian Plate and Asian plates at shallow depths along the Himalayan front. The Tibetan Plateau is a very large uplifted area. It is not as active as Himalaya. Fitch (1970) observed strike-slip movement on either a N-S or an E-W striking plane for one event in northeastern part of the Tibetan plateau. The P axis trends northeast, perpendicular to the local trend of the Tsaidam basin. Solutions for two events in the Tibetan Plateau show normal faulting with both nodal planes striking approximately north. The earthquakes reflect the vertical movement of the Tibetan plateau. The solutions are consistent with block faulting that is common in this region. The intermediate depth earthquakes of the Hindu Kush are all pure thrust. The solutions in the region between the Hindu Kush zone and Lake Baikal are mostly of the thrust type. The earthquakes within central Asia do not show any definite pattern and there are events of all three types: strike-slip, normal and thrust. The Burmese arc is very complicated. Here we get all three types of faulting. South of 24”N one normal and two thrust fault plane solutions indicate under-thrusting of the Indian plate towards the east along the Burmese arc. Singh and Gupta (19.80) have studied the direction of rupture propagation for two earthquakes at the Burma-India border regions and the result shows the southward movement of the Burmese block with respect to the Indian plate, and the contortion of the descending lithosphere of the Indian plate at a depth of 134 km. Two shallow focus events occurring near the southeastern flank of the Syntaxis have a strike-slip mechanism solution. One event having a 22.4”N, 93.6”E epicentre shows normal faulting with the axis of tension nearly horizontal and approximately perpendicular to the trend of the Burmese arc. The thrust faulting mechanisms from both the Assam and Burma arcs indicate that the axis of maximum compressive stress is more nearly horizontal than vertical and it trends perpendicular to the strike of the mountain arcs. From the two intermediate depth earthquakes in the Karakorum mountains near Hindu Kush underthrusting of the Eurasian plate to the southwest is inferred. The earthquakes occurring in the Nepal and Tibet Himalaya show thrust faulting. For these solutions the slip direction of the underthrusting block along the nodal plane dipping north northeast beneath the mountain front, is in approximately the northeastern direction. Singh et al. (1979) have studied the source characteristics of 33 earth-

107

Fig. 5. Plot of earthquake epicentres (dots) used in source parameter studies. The fault plane orientations which were used in the radiation pattern function determination are shown. (Adapted from Singh et. al., 1979.)

quakes (Fig. 5) with magnitude mb between 4.4 and 6.0, which occurred in the Himalayan and nearby regions, using the records of the Hyderabad seismograph station. The P and S waves of these events are interpreted in terms of Brune’s seismic source model for estimating the source parameters. The results of the source parameter estimates show high stress drops and high apparent stress (from a few bars to several tens of bars in general) in the regions of Himalaya, Burma, the Tibet plateau and the Kunlun fold belt. In the Kunlun area the stress drop and apparent stress are found to be higher than the other regions. Molnar et al. (1973) have inferred high shear stresses for the region for the following reasons. (1) The extremely high accelerations caused by the Assam earthquake of 1897. (2) According to the revised list of largest earthquakes between 1897 and 1957 (Gutenbe~ and Richter, 1949), 7 out of the 22 largest earthquakes occurred in central Asia. (3) The relatively large energy content in the high-frequency portion of the body wave spectrum. The unequal distribution of stresses in these regions can be explained in terms of the continental collision of the Indian and Eurasian plates. It has also been observed that the earthquakes which occur along the plate boundary are characterized by a low stress drop since they result simply from the movements on the pre-existing zones of weakness, while the intra-plate events show a high stress drop since they result from the creation of new faults requiring higher stresses. Also, locally the shear stress in a particular area may be higher or lower than the regional stress because of the material inhomogeneities, complex fault geometry, and previous seismic or aseismie stress released in the area (Thatcher and Hanks, 1973).

108

GRAVITY

STUDIES

For the region extending from Kashmir Himalaya to Nepal Himalaya including the adjacent alluvial plains in the south, Survey of India has compiled a Bouguer anomaly map, and Airy Heiskanen (2’ = 30 km) and Pratt Hayford (D = 113.8 km) isostatic anomaly maps. As may be seen in Fig. 6 the Bouguer anomaly decreases from about -50 mgal in the plains to about -300 mgal over the high Himalaya, The isostatic anumaty maps exhibit the same trend -showing negative values over the alluvial planes, attaining a zero value over sub- and lower Himalaya, and then rising to a positive value of 80-100 mgal in the high Himalaya. Qureshy (1969) and Qureshy et al. (1974) mention that isostatic equilibrium prevails in the Himalaya. From the study of deflections from the vertical and the nature of the geoid in the Himalaya, Chugh and Bhattaeharji (1973) have also concluded that isostatic equilibrium prevails in the Himalayan region. Results reported by Kono (1974), however, are different. He has carried out gravity measurements at 145 points in eastern Nepal and studied the data by dividing into three zones, i.e, Higher Himalaya, Lower Ihmalaya, and the Foothills. Bouguer anomalies in these three zunes exhibited no correlation whatsoever with the elevations. This fed him to conclude that isustasy does not prevail in the

‘ig. 6. Bauguer gravity anomaly

map of the Himalayan

regian by Survey of India.

109

Himalaya in the wavelength of less than 100 km. Kono (1974) further observed that Bouguer anomalies projected on a line perpendicular to the structural trend of the Himalaya in this region shows a remarkable linear relation with the distance measured along the line. Assuming a crustal thickness of about 30 km in the foothills, he estimated that it continues to increase reaching a maximum thickness of 74 km in the Tibet Plateau region at a distance of about 250 km from the Main Boundary Thrust. Whereas the great Himalaya chain with an average height of 5 km or more lies only 150 km north of the Main Boundary Thrust, Kono (1974) concludes that if Himalaya were to be gravitationally stable, the crustal thickness should have gained a maximum value under the highest elevations. CONCLUSIONS

Surface wave dispersion studies have indicated an extension of the Indian shield below the Indo-Gangetic plains and under the Himalaya Tibetan region. The crust-al thickness under the region increases towards the north and it is found to be of the order of 70-80 km below Tibet. There is conflicting evidence regarding the present plate boundary between the Indian and the Eurasian plates. The ERTS imagery, the seismicity, the shield type of the upper mantle structure beneath Tibet, and the transverse faults over the Himalayan ranges support the inclusion of Tibet as a part of the Indian plate with its boundary along the Tien Shan fold belt (Fig. 2). The limited DSS work in the Kashmir Himalaya indicates a crustal thickness of the order of 70 km and block faulting and uplift. On the other hand, the focal mechanism and stress drop studies indicate under-thrusting of the Indian block under the Himalaya and convergence of the Indian and Asian plates along the Himalayan front. The need to study the Himalayan-Tibetan region in greater detail to understand the geodynamics of this part of the globe cannot be overemphasized. There is need to establish a number of seismological stations to record earthquakes of magnitude 4.5 and below and to determine their locations and focal mechanism more accurately. Detailed geological, geophysical, and geochemical studies in selected critical parts of the region are required to resolve many controversies and to provide solutions to problems relating to applicability of plate tectonics to continental masses. REFERENCES Aliev, S.A., Beliaevsky, N.A., Butovskaya, E.M., Volvovsky, B.S., Volvovsky, I.S., Krasnopvtseva, G.V., Pak, V.A., Polshkov, M.K., Ruhailo, V.I., Sollogub, V.B., Talvirsky, B.B., Tregub, F.S., Khamrabayev, I.Kh. and Kharechko, G.E., 1976. The seismic experiment in the northern Pamirs. In: C.L. Drake (Editor), Geodynamics: Progress and Prospects. American Geophysical Union, Washington, pp, 128-136. Bartole, R., Ebblin C. and Marussi, A., 1980. The earth’s crust along the KarakulZonkul-Nanga Parbat-Lawrencepur DSS profile according to structural and geophysical Data. Monogr. Pamir-Himalaya, in press.

110 Bird, P. and Toksoz, 266: 161-163.

M.N.,

1977.

Strong

attenuation

of Rayleigh

waves in Tibet.

Brune, J.N., 1968. Seismic moment, seismicity and rate of slip along major J. Geophys. Res., 73: 777-784. Chugh, R.S. and Bhattacharji, J.C., 1973. Study of isostasy in Himalayan sented at Symp. Himalayan Geology, Nainital, India.

Nature,

fault zones, region.

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Chun, K.Y. and Yoshii, T., 1977. Crustal structure of the Tibetan Plateau: A surface wave study by a moving window analysis. Bull. Seismol. Sot. Am., 67: 735-750. Finetti, I., Giorgetti, P. and Poretti, G., 1980. The Pakistani segment of the DSS-profile. Nanga Parbat-Karakul. Monogr. Pamir-Himalaya, in press. Fitch, T.J., 1970. Earthquake mechanisms in the Himalayan, Burmese and Andaman regions and continental tectonics in Central Asia. J. Geophys. Res., 75: 2699-2709. Gansser, A., 1974. Himalaya Mesozoic-Cenozoic Orogenic Belts. Geological Society, London, p. 267. Gupta, H.K. and Narain, Hari, 1967. Crustal structure of the Himalayan and Tibet Plateau regions from surface wave dispersion. Bull. Seismol. Sot. Am., 57 : 235~248. Gupta, H.K., Nyman, D.C. and Landisman, M., 1976. Rayleigh wave group velocities between New Delhi, India and Shiraz, Iran, extending to long-periods. In: C.L. Drake (Editor), Geodynamics: Progress and Prospects. Am. Geophys. Union, Washington, D.C., pp. 121-126. Gupta, H.K., Nyman, D.C. and Landisman, M., 1977a. Shield-like Upper Mantle structure inferred from long-period Rayleigh and Love wave dispersion investigations in the Middle East and South East Asia. Bull. Seismol. Sot. Am., 67: 103-119. Gupta, H.K., Nyman, D.C. and Landisman, M., 1977b. Shield like upper mantle velocity Inferences drawn from long-period surface structure below the Indogangetic Plains: wave dispersion studies. Earth Plan. Sci. Lett., 34: 51-55. Gutenberg, B. and Richter, C.F., 1949. Seismicity of the Earth and Associated Phenomena. Princeton University Press, Princeton, N.J., 1st ed., 310 pp. Hamada, K., 1972. Regionalized shear-velocity models for the upper mantle inferred from surface wave dispersion data, J. Phys. Earth., 20: 301. Kaila, K.L. and Narain, Hari, 1971. A new approach for preparation of quantitative seismicity maps as applied to Alpine Belt-Sunda Arc and adjoining areas. Bull. Seismol. Sot. Am., 61: 1275-1291. Kaila, K.L. and Narain, Hari, 1976. Evolution of the Himalaya based on seismotectonics and deep seismic soundings. Himalayan Geology Seminar, pp. l-30. Kaila, K.L., Krishna, V.G., Chowdhury, K. Roy and Narain, Hari, 1978. Structure of the Kashmir Himalaya from deep seismic soundings. J. Geol. SOC. India., 19: l-20. Knopoff, L., 1976. Regionalization of the Arctic region, Siberia and Eurasian continental area. Final Report, IGPP, University of California, 53 pp. Kono, Masaru, 1974. Gravity anomalies in east Nepal and their implications to the crustal structure Le Pichon,

of the Himalayas. Geophys. J. R. Astr. Sot., 39: 283-299. X., 1968. Sea floor spreading and continental drift. J.

3661-3697. Molnar, P. and Bruke,

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463. Molnar, P., Fitch, T.J. and Wu, F.T., 1973. Fault plane solutions of shallow earthquakes and contemporary tectonics in Asia. Earth Plan. Sci. Lett., 19: 101-112. Peterson, J. and Orsinim, N.A., 1976. Seismic research observatories: Upgrading the world-wide seismic network. EOS, Am. Geophys. Union, 57 : 548-556. Qureshy, M.N., 1969. Thickening of a basalt layer as a possible cause for the Uplift of the Himalayas - a suggestion based on gravity data. Tectonophysics, 7 : 137-I 57. Qureshy, M.N., Venkatachalam, S. and Subramanyam, C., 1974. Vertical tectonics in the Middle Himalayas: An appraisal from recent gravity data. Geol. SOC. Am. Bull., 85: 921-926.

111 Rastogi, B.K., 1976. Source mechanism studies of earthquakes and contemporary tectonics in Himalaya and nearby regions. Bull. Int. Inst. Seismol. Earthquake Eng., Tokyo, 14: 99-134. Scarascia, S., Colombi, B., Guerra, I. and Luongo, G., 1980. Preliminary report on seismic measurements along the profile Lawrencepur-Astor. Monogr., Pamir-Himalaya, in press. Singh, D.D. and Gupta, H.K., 1980. Source mechanism studies of two Burma-India border earthquakes. Seminar in Seismology, Kurukshetra, University, India, Bull. N. Indian Sot. Earthquake Technol., in press. Singh, D.D., Rastogi, B.K. and Gupta, H.K., 1979. Spectral analysis of body waves for earthquakes in the Himalaya and nearby regions and their source parameters. Phys. Earth Planet. Int., 18: 143-152. Thatcher, W. and Hanks, T., 1973. Source parameters of Southern California earthquakes. J. Geophys. Res., 78: 8547-8576. Tung, J.P., 1975. The Surface Wave Study of Crustal and Upper Mantle Structure of Main Land China. Ph. D. Thesis, University of Southern California, 247 pp. Valdiya, K.S., 1976. Himalayan transverse faults and folds and their parallelism with subsurface structure of North Indian Plains. Tectonophysics, 32: 353-386.